1. Field of the Invention
The present invention relates to a local area network (LAN), more specifically to the optimization of communication quality for realizing an optimal communication environment between a wireless LAN satellite station and a master station in a wireless LAN system mainly using an electromagnetic wave in a millimeter wave range.
2. Description of the Related Art
These days a LAN has become popular and the amount of data handled on a LAN has been also increasing. On the other hand, data terminals including a personal computer have been improved in communicating functions, and have been miniaturized. Under these circumstances, the need for data communication in a mobile environment is advocated, and attention is being paid to a wireless LAN system.
So far an electromagnetic wave in an ISM range, that is, approximately the 1 to 3 GHz range, has been used for a wireless LAN system. However, since this bandwidth is also used for industrial purposes or electronic ovens, a large amount of noise is generated. To suppress this large amount of noise it is necessary to employ a spread spectrum communication method, which has made a system complicated. The system also has a drawback that in this wave range a bandwidth required for a high speed transmission cannot be secured.
For this reason, attention is currently being paid to a millimeter wave range of 50 to 70 GHz which is still an unused wave range for data communication.
An electromagnetic wave in this wave range is characterized in that it is strong to keep straight on, and it is easily absorbed by oxygen and glass. For example, since there is little possibility that an electromagnetic wave leaks outside when it is used in an office environment, it is effective in security. Furthermore, since it is an unused band width, and thereby there is no need to employ a spread spectrum communication method, the system can be simplified, compared with the case when an ISM range is used. Since in a millimeter wave range a band with required for a high speed transmission can be secured, it is a very promising wave range at present when a high speed transmission of over 100 Mbps is becoming popular in a wired LAN system.
However, the wireless LAN system using an electromagnetic wave in a millimeter range has the following problems.
The first problem is that a communication environment rapidly deteriorates due to a subtle change of the position and direction of an antenna of a satellite station. In order to expand a communication-available area in a millimeter wave wireless LAN system an antenna with a rather broad radiating directivity is usually used for the master station. However, a millimeter wave is strong to keep straight on and is easily absorbed by oxygen. There is also influence from interference among satellite stations and multipath interference which is caused by being a plurality of routes for an electromagnetic wave transmitted from a satellite station. For this reason, to secure a favorable communication environment, it is necessary to sharpen the radiation characteristic of the antennae of satellite stations and to always direct the radiation characteristic of the satellite stations exactly to the antenna of the master station.
To avoid the influence from interference among satellite stations and multipath, the use of diversity antennae and the introduction of a spread spectrum communication method can be considered. However, it is technically difficult to implement these methods in a millimeter wave range, and even if it can be implemented, the system becomes complicated. This is the second problem.